Aliphatic secondary amine oxidation inhibitors



3,493,512 ALIPHATIC SECONDARY AMINE OXIDATION INHIBITORS Robert T. Trites, Cincinnati, Ohio, assignor to Emery gldustries, Inc., Cincinnati, Ohio, a corporation of bio No Drawing. Continuation-impart of application Ser. No. 409,291, Nov. 5, 1964. This application Feb. 27, 1967, Ser. No. 618,996

Int. Cl. Cm 3/ 26; C07c 87/123 US. Cl. 252-515 6 Claims ABSTRACT OF THE DISCLOSURE An oxidation inhibitor for lubricants particularly synthetic ester lubricants comprising secondary amines having the following general structure:

wherein R and R are branched chain, saturated aliphatic hydrocarbon groups of from to 24 carbon atoms, and are the hydrocarbyl or non-carboxyl portion of normally liquid, branched chain, saturated, aliphatic, monocarboxylic acids containing from 16 to 25 carbon atoms.

BACKGROUND OF THE INVENTION This invention is concerned with oxidation inhibitors for lubricant systems and more particularly with branched chain, saturated, aliphatic secondary diamines which are useful as oxidation inhibitors for synthetic ester lubricants.

Oxidation inhibitors are added to lubricant fluids to minimize oxidative decomposition of the fluid to acid products which tend to form sludge and to corrode the various metal surfaces with which the lubricant comes into contact. The demand imposed upon oxidation inhibitors is closely related to the engine operating temperature with an increase in engine temperature placing an in creased demand upon the oxidation inhibitor. Thus, many oxidation inhibitors which give adequate protection at temperatures around the 300 F. level have substantially no utility at higher temperatures. With an increase in the sophistication of gas turbine engines has come an increase in the operating temperatures, the increase being from about 300 F. maximum temperature in the gas turbine engines utilized in aircraft propulsion in 1953 to temperatures of 400450 F. in modern gas turbine aircraft engines. At temperatures above 300 F., the conventional oxidation inhibitors, such as alkylated phenols and conventional petroleum lubricating oils fail to perform their respective functions.

The secondary dialkylarnine oxidation inhibitors of the present invention are capable of inhibiting oxidation of lubricant compositions at temperatures as high as 450 F. They have the further advantages that they are readily soluble in both hydrocarbon and synthetic lubricants and thus compatible with either type of lubricant system, they do not appreciably affect the liquidous characteristics of the base fluid, and they do not attack the rubber of elastomer seals in turbine aircraft engines. One of the most diflicult problems connected with the oxidation inhibitors heretofore developed which have been able to withstand high temperatures has been their tendency to attack the rubber or elastomer seals causing them to become hard and brittle and cease to perform. Some seals such as Viton-A, a fluorinated polymer, are depolymerized by many of the so-called high performance oxidation inhibitors.

In the past, probably the most popular oxidation in- Patented Feb. 3, I970 hibitors for high temperature engine operations have been aromatic secondary amines such as p,p'-dioctyl-diphenylamine. While these aromatic secondary amines have been able to withstand high temperatures and have generally been stable antioxidants, they have suffered from the disadvantage that they are dirty, that is, they create a sludge which clogs filters and causes lubricant starvation when used at high temperatures, i.e. 450 F. One of the outstanding advantages of the branched aliphatic secondary amines of the present invention is that they have outstanding cleanliness characteristics, enabling the engines to operate free from clogging caused by sludge accrual.

The aliphatic secondary amine oxidation inhibitors of this invention have, as noted earlier, excellent solubility characteristics and are miscible in all proportions with ester type lubricants, polyglycol lubricants and petroleum base oils without appreciably affecting the liquidous range of the base lubricants. They are also highly stable and are adapted to be employed with any of the conventional additives which are used in formulating a lubricant intended for high temperature service. Further, they remain liquid at extremely low temperatures, i.e. temperatures as low as F., and thus they are useful in lubricants which must exhibit an extremely wide liquidous range. Some aliphatic secondary amines (dialkylamines), such as distearylamine, dimyristylamine, and dilaurylamine, have been used as oxidation inhibitors for petroleum lubricants in the past; however, they are generally unsatisfactory for use with ester lubricants because they are solids at room temperature and fall out of solution due to crystallization at low temperatures, i.e. temperatures below about 20 F. Furthermore, after having precipitated out at low temperatures, they are difficult to get back into solution and are frequently trapped by filters, resulting in clogging of the lubrication system.

DESCRIPTION OF THE INVENTION The oxidation inhibitors of this invention are secondary amines prepared from liquid branched chain higher fatty acids and have the general formula:

wherein R and R are both branched chain, saturated aliphatic hydrocarbon groups of from about 15 to 24 carbon atoms, each group being in effect the hydrocarbyl or non-carboxyl portion or normally liquid, branched chain, saturated, aliphatic, monocarboxylic acids containing from about 16 to 25 carbon atoms. The branched chain monocarboxylic acids employed in the formation of the secondary amines of this invention are liquid at room temperature and are prepared by synthetic methods. An acid particularly suited for use in the preparation of the secondary amines which form the basis of this invention is one containing 18 carbon atoms formed as a by-product in the polymerization of naturally occurring unsaturated fatty acids in accordance with the method described in US. Patent 2,812,342 (hereinafter referred as the 342 patent and identified as isostearic acid). The 342 patent disclosure is incorporated herein by reference. According to the disclosure of the 34-2 patent, monounsaturated or polyunsaturated fatty acids are treated thermally in the presence of water with or without a catalyst to produce products which are known commercially as dimer acids. This process inherently produces a substantial amount of by-product acids which are a mixture of monomeric acids. The monomer mixture, which is normally distilled in vacuo from the polymercontaining reaction product, includes saturated fatty acids which have not been affected by the polymerization treatment, probably some unsaturated fatty acids which have not been affected by the polymerization treatment and fatty acids which have been structurally modified by the polymerization treatment in such a manner that they resist further polymerization.

Following the procedure outlined in the 342 patent, the monomeric fatty acids are hydrogenated to reduce the iodine value of the mixture to a level below and perhaps as low as 3, thereby reducing any natural unsaturated fatty acids which may be present to saturated fatty acids. The hydrogenation treatment also reduces the iodine value of the fatty acids which have been structurally modified in some degree by the polymerization treatment. Further modifications may take place during the hydrogenation although it is not known to what extent structural modification of the acids takes place during that step. The resulting hydrogenated fatty acid mixture is then solvent separated to remove the normal solid fatty acids, such as stearic and palmitic acids. The remaining saturated fatty acid is a modified product which, while containing 18 carbon atoms, possesses a branched chain structure, a titre below C. and an iodine value of substantially 3 to 10 and is referred to as isostearic acid. The exact structure of isostearic acid is not known; however, nuclear magnetic resonance tests have indicated that it is branched and has an average of at least 3 alkyl groups of 2 carbon atoms or more per molecule, indicating the presence of at least two side chains.

In manufacturing the isomeric C acids for use in preparing the secondary amines of this invention, the process described in the 342 patent must be practiced very carefully to effect the best possible segregation of the different monomeric acids. For instance, if the hydrogenation process is practiced inelfectively, oleic acid will be present as an impurity in the end product and being unsaturated would be totally unacceptable as a component of an oxidation inhibitor. Further, if the solvent separation operation is not practiced eifectively, stearic or other solid acids will be present and when converted to amines in accordance with the process of this invention and used as antioxidants will give the lubricant an unacceptably high pour point. In summary, the process of the 342 patent must be practiced with sufficient care that structurally modified acids are recovered without the presence of any objectionable quantity of undesirable impurities.

The 342 patent describes the branched chain C fatty acids (isostearic acids) as structurally modified acids, (which they are in relation to the starting materials); however, these acids are more aptly described as structurally stabilized acids. The acids prepared by the method of the 342 patent receive two severe treatments, one a polymerization treatment which tends to both polymerize and to modify structurally the unsaturated acids, and the other a hydrogenation treatment which saturates and perhaps further modifies the structure of the acids. Hence, the structurally modified acids have become stabilized in the sense that they have been subjected to such rigorous treatment that they have developed their most stable structural form and thus are aptly described as structurally stabilized.

The isostearic acid described above and in the 342 patent, forms, when converted into a secondary amine of this invention, the preferred embodiment of the invention.

Another normally liquid monocarboxylic acid which may be used in the preparation of the secondary amines of this invention is one which contains 22 carbon atoms and is prepared by the polymerization of isobutylene with oleic acid. The resulting product, on being hydrogenated to effect saturation, will consist essentially of a mixture of 9- and 10-secondary butylstearic acids. Still other liquid branched acids which may be used as 9-n-octylmargaric acid and IO-methylstearic acid. As

previously noted, the acids which may be used in the formation of the secondary amines of this invention should have from 16 to 25 carbon atoms. The secondary amine oxidation inhibitors derived from these acids must be fully compatible at use levels with the lubricant system and the lubricant must be liquid at a temperature of F.

The secondary amines of the present invention may be prepared by reacting the saturated liquid aliphatic branched acids with ammonia to convert to the nitrile of the liquid branched acid use. The nitrile is recovered and hydrogenated to the amine form and the secondary amine is separated and used as the oxidation inhibitor of the present invention. One method for preparing the oxidation inhibitors of the present invention is as follows: The liquid branched aliphatic acid is heated to a temperature of from about 250 to about 350 C. (preferably 320-330 C.) and ammonia is passed constantly into contact therewith with the water of reaction being removed. The reaction should be conducted at a pressure of from about 50 to 150 p.s.i. (preferably to p.s.i.) and should be continued until the reaction product is substantially a nitrile. After the nitrile has been recovered, it is hydrogenated using a standard hydrogenation catalyst such as nickel on keiselguhr, Raney nickel, palladium or platinum on suitable support, or other well known hydrogenation catalysts. The hydrogenation should be effected when using a nickel catalyst at pressures of from 50 to 400 p.s.i., preferably 300 to 400 p.s.i., and at temperatures of to 250 C., preferably 200 to 225 C. During the hydrogenation process the nitrile is first converted to a primary amine and then in substantial part to a secondary amine as ammonia is split out. The reaction is as follows:

2RNH2 RNR NHzI where R is the hydrocarbyl residue of the liquid branched aliphatic acid. After the nitrile has been substantially converted to the amine form, the amine product is cooled, filtered, and fractionated to separate the secondary amines from the primary amines and tertiary amines.

The secondary amine oxidation inhibitors of this invention may be used with a large number of lubricating oils, both mineral oils and synthetic oils including a wide variety of aliphatic, carboxylic acid esters which have been proposed for lubricant usage. Among these lubricants are the dialkyl (C -C esters of aliphatic dicarboxylic acids (C C the principal diesters being those derived from adipic, azelaic, or sebacic acids and a C to C alcohol.

Specific examples of ester lubricants in which the oxi dation inhibitors of this invention may be used are dioctylazelate, di-Z-ethylhexylsebacate, di-2-ethylhexyladipate, dilaurylazelate, disecondaryamylsebacate, di-2-ethylhexylsuccinate, di 2-(2-methoxyethoxy)ethylsebacate, di 2-(2-ethylbutoxy)ethylsebacate, di-Z-ethylhexylazelate.

In addition to the aliphatic dicarboxylic acid esters described above, the secondary amine oxidation inhibitors of this invention may be used with polyester lubricants formed by the reaction of an aliphatic dicarboxylic acid, a glycol, and a monofunctional compound, which is either an aliphatic monohydroxy alcohol or an aliphatic monocarboxylic acid, in specified mole ratios. The instant oxidation inhibitors may also be used with complex esters formed by such reactions as that of a mixture containing specified amounts of 2-ethyl-l,3-hexanediol, sebacic acid and Z-ethylhexanol or by the reaction of a mixture containing adipic acid or azelaic acid, diethyleneglycol and 2 ethylhexanoic acid. Esters formed by reaction of a monocarboxylic acid and a glycol may also be used as the ester component. The acid component is usually an aliphatic acid containing at least 6 carbon atoms. The glycol component can be a straight glycol such as 1,6-hexanediol but other glycols such as tetraethyleneglycol or dipropyleneglycol may also be used. Specific examples of the diesters of glycols are the following: di-n-decanoate of 1,4-butanediol, di-2-ethylhexanoate of 1,6-hexanediol, dilaurate of 1,4-hexanediol, and the dioctanoate of 1,5 pentandiol. Esters formed by reacting trimethylolalkanes (C C with various monobasic acids, primarily C C acids, comprise another example .of esters useful as the base fluid of lubricants which can be used with the compounds of this invention.

The secondary amine oxidation inhibitors of this invention are capable of withstanding the severe Pratt and Whitney 450 F. oxidation test. They out-perform at 450 F. p,p-dioctyldiphenylamine, one of the most effective oxidation inhibitors heretofore developed for use in high performance (high temperature) engine operation.

The amount of secondary amine oxidation inhibitor to be employed in a given lubricant, will, of course, vary with the particular antioxidant chosen, the nature of the lubricant fluid, and the engine conditions to be encountered. Generally speaking, good results are obtained using amounts of from about 0.15.0% by weight of the secondary amine with the preferred range being from 0.5-2.0% by weight.

The oxidation inhibitors of this invention may be used with a number of other lubricant additives as well and in many instances it is preferred or necessary that additional materials be used. One additive which generally must be included is a metal deactivator such as PANA (phenyl-alpha-naphthyl-amine). Other conventional additives including rust inhibitors or materials which impart load carrying qualities, auxiliary oxidation inhibitors, if desired, and V.I. improvers may be used.

The present invention is further illustrated by the following examples which it should be understood are illustrative and not limitative of the scope of the invention.

EXAMPLE I An oxidation inhibitor of this invention, diisostearyl amine, was prepared in the following manner: sixty-five pounds of isostearic acid were charged into a 12 gallon autoclave equipped with stirrer. The acid was heated slowly to 320 C. under a constant flow of ammonia and with the use of a constant vent to remove the water of reaction. The pressure was maintained at about 90 to 100 p.s.i. After 7 hours the reaction products were analyzed and it was found that a considerable amount of the product was still in the amide form. The reaction was thus continued under the above-noted conditions for an additional 7 hours after which the product was almost exclusively isostearylnitrile. The reaction was cooled and the product filtered through a No. 27 full-flow filter. Fifty-four pounds of isostearylnitrile were recovered.

ples were taken to determine the neutralization equivalent values. When it had been determined that the product had been substantially hydrogenated to the amine form (3 /2 hours), the system was cooled to 150 C. The product, approximately 1000 grams, was blown from the autoclave and filtered using a filter aid (dicalite). The neutralization equivalent of the filtered product was 645. The filtered material was then topped at a pressure of 0.3 to 1.2 mm. of Hg and at a temperature up to about 260 C. to remove all of the primary amine. The residue 83.2% by weight was then given a distillation using a column of short length, under 0.3-0.4 mm. Hg pressure. The first fraction of the resulting product was removed at a pot temperature of 275280 C. and a vapor ternperature of 231 C. under a pressure of 0.3 ml. of mercury. This first fraction represented 1.3% by weight of the product and had a neutralization equivalent of 568. A second fraction which was diisostearylamine was then removed at a pot temperature of 280-300 C. and a vapor temperature of 258 to 270 C. under a pressure of 0.35 mm. of mercury. This fraction represented 72.2% by weight of the product prepared as described above, had a neutralization equivalent of 522, and was the fraction used as the oxidation inhibitor in the examples of this invention.

EXAMPLES II-V Comparative oxidation tests were conducted upon a number of oxidation inhibitors, N-ethylaniline, bis(1, ethyl 3 methylpentyl)amine, p,p'-dioctyldiphenylamine, all commercially used amine oxidation inhibitors and diisostearylamine, an oxidation inhibitor of the present invention. The test procedure was as follows: 100 grams of a lubricant, trimethylolpropane dipelargonate monoheptonoate, containing an oxidation inhibitor in the amount indicated in Table I and a weighed copper coupon were placed in an Erlenmeyer flask. The flask was immersed in a silicone bath so that the surface ofthe sample was about one inch below the surface of the liquid bath.

An air tube was inserted into the flask and kept one inch above the surface of the sample. The air rate was adjusted to 16.8 cubic feet per hour. A 1 ml. sample was withdrawn by pipette periodically and analyzed. The contents of the pipette were drained through a 200 mesh screen and a 100 F. kinematic viscosity test was made using a 1 ml. Cannon Micro Viscometer (actual capacity is approximately 0.5 ml.). At the end of the test (after four days), the remainder of the sample was filtered through a 10 micron millipore filter. The copper coupons were weighed after being washed thoroughly with a solvent and weight loss was calculated as mg./cm. of surface exposed. The results of the tests run upon the various amine oxidation inhibitors are shown below in Table I.

TABLE I Example 1.0% dioctyl- 2.5% bis 1.5% N-ethyldiphenyl- (1,-ethyl-3,- 2.5% diisoanillne, amine, 1.0% methylpentyl)- stearylarnine, Viscosities cs. at 100 F. 0.5% PANA PANA amine, 0.5% PANA 1.0% PANA Original 20. 1 22. 1 19. 4 21. 7 1 day 24- 8 24. 5 25. 7 23. 5 2 days 35. 9 26. 9 36. 8 25. 4 3 days". 50.0 30. 3 51. 3 28. 8 4 days 72. 3 35. 8 77. 1 33. 2 Copper wt. loss, In 0. 78 0. 23 0. 33 0. 03

1000 grams of the isostearylnitrile and 20 grams of 1% nickel catalyst, Harshaw Nickel 0104-P (50% active nickel on keiselguhr) were placed in a rocking autoclave. The system was vented with hydrogen to remove air and then pressurized to 300 p.s.i. with hydrogen. Heat was applied until the temperature reached 200 C. and the reaction was continued at this temperature and at a As may be seen from the above test, the viscosity of diisostearylamine, the oxidation inhibitor of the present invention, was less affected over a period of time than any of the other amine oxidation inhibitors tested. N-ethylaniline and bis(1,-ethyl-3,-methylpentyl)amine show a viscosity rise far in exces of that which can be tolerated. It may also be noted that the copper corrosion pressure ranging from 290-380 p.s.i. Intermediate samwas substantially less with diisostearylamine.

7 EXAMPLES v1 AND v11 In order to further compare the oxidation inhibiting cloud point and pour point with the results as shown below.

properties of diisostearylamine and p,p-dioctyldiphenyl- Diisodecylazelate 99% by weight 99% by weight. amine, both compositions were subjected to the Pratt and gig gg gggfffigg 1% by weght by Wei ht Whitney 450 F. oxidation test. The test was conducted Cloud point 60 +0 F. g in the following manner: 200 ml. of test solution com- Pour pomt Less than prising as the lubricant trimethylol propane dipelargonate, monoheptanoate, 1.0% by weight phenyl alpha naphthyl It may regdlly f from the.above i that amine, a copper deactivator and 25% by Wight of sostearylamine 1s superior as a lubrlcant additive that it oxidation inhibitor was prepared. One inch by one inch compatkble Wlth the ester lubnfiant P mperature coupons of copper, Ste 61, Silver, titanium, magnesium, and rangzlfar in excess of that at which dioctyldecylamine 1s aluminum were wired in sequence and suspended in the calia Performmg' test liquid to within /1," of the bottom of the test solution. 1 2 An air condenser was then placed above the test liquid 15 f OX1 anon m 1 a lubncant havmg the and the entire apparatus was placed in a constant tem- Ormu perature heating block controlled to 450 F. plus or R -GHz minus 1 F. Dry air was then introduced through the inlet tube at a rate of 5 liters per hour for 48 hours. At the end of the test period, the samples were cooled to R2 CH2 room temperature. The metal coupons were removed'and wherein R and R2 are branched chain aliphatic h d SOIVent Cleaned to remove 501111916 deposits, then Weighed carbon radicals having from 15 to 24 carbon atoms and to doetel'mlne COITOSKPII lossesi vfscoslty at 100 and are the hydrocarbon residue of liquid saturated branched 210 F. was determined. The viscosity changes are exli h i id Pressed as Percentage in Table The p were then 2. The oxidation inhibitor of claim 1 wherein R and 12:11:? 6closa)rsgyfiggflipilgre gl e ggg i t-s vi g agsggd R are branched aliphatic hydrocarbon residues of isostearic aci Wlth a e gng 511 11?1 i i i after'wllllch the collectgd-sludge 3. A lubricant composition comprising a major portion 3:: ggglgmti .samgles uTtgee 253g vtgluaere Iexl zressgf tills c5 f0a7 synthetichestzr lubricait togetlllfr with from 0.1 to

c a ge e s 0 y Weig t o the oxi ation in ibitor of claim 1. solution was determined by titration of the tested lubri- 4, A l bri t composition comprising a major portion 125 22% 553? 32.5123022112 8333. iliiiii d 3; 2i); i. -hi ii ii ifi. with i f 0 y werg o t e OX1 ation in i itor o c aim 3. weight difference (plus or minus) in the metal coupons 5-. The method which comprises lubricating moving before and after the tests and are reported as mg. per parts of an engine with a synthetic ester lubricant containcm. of surface exposure. ing the oxidation inhibitor of claim 1.

TABLE II Example Lube comprising Lube comprising trimethyloltrimethylolpropane dipelargonate mono heptanoate, 1%

propane dipelargonate mono heptanoate, 2.5

by wt. p,p-diwt. percent octyldiphenyldiisostearyl- P and W Results amine, 1% PANA amine, 1% PANA spec.

Viscosity change 100 F., percent 229. 8 25.6 max. 4. 44 3. 69 Report Total sludge (mg.) 2, 750 538 Metal corrosion (mg Copper 2. 86 0. 24 :!:0. 30 Steel +0. 02 +0. 15 :i:0. 30 +0. 02 +0. 14 :|:0. 30 +0. 02 +0. 13 :|:0. 30 -11. 85 +0. 11 :|:0. 30 Aluminum +0. 03 +0. 12 :l:0. 30

The superiority of diisostearylarnine as an oxidation inhibitor is evident upon review of the above data. It may be seen that the diisostearylamine generates far less sludge, suppresses the acid value change, and metal corrosion remains within tolerable limits in each instance whereas excessive copper and magnesium corrosion are experienced using p,p-dioctyldiphenylamine.

EXAMPLES VIII AND IX In order to demonstrate the diiference between a secondary amine oxidation inhibitor of the present invention, diisostearylamine and a linear diakylamine of comparable molecular weight, dioctyldecylamine, comparative tests using these two materials were conducted. The tests were performed by adding to one of two diisodecylazelate samples 1% by Weight of diisostearylamine and to the other sample 1% by weight of dioctyldecylamine. After addition of the diarnines to the respective samples, they were heated to a temperature of 100 C. At this temperature each of the amines were blended completely into the ester solution, They were then allowed to cool to test for 6. The method which comprises lubricating moving parts of an engine with a synthetic ester lubricant containing the oxidation inhibitor of claim 2.

References Cited DANIEL E. WYMAN, Primary Examiner W. I. SHINE, Assistant Examiner US. Cl. X.R. 

